Electricity transmission and distribution networks (or grids) must balance the generation of electricity with the demand from consumers. This is normally achieved by modulating the generation side (supply side) by turning power stations on and off, and running some at reduced load. As most existing thermal and nuclear power stations are most efficient when run continuously at full load, there is an efficiency penalty in balancing the supply side of the electricity system in this way. The expected introduction of significant intermittent renewable generation capacity, such as wind turbines and solar collectors, to the networks will further complicate the balancing of the grids, by creating uncertainty in the availability of parts of the generation fleet. A means of storing energy during periods of low demand for later use during periods of high demand, or during low output from intermittent generators, would be of major benefit in balancing the grid and providing security of supply.
Power storage devices operate on a highly intermittent basis when there is a shortage of generating capacity on the transmission or distribution network. This can be signaled to the storage device operator by a high price for electricity in the local power market or by a request from the organisation responsible for the operating of the network for additional capacity. In some countries, such as the United Kingdom, the network operator enters into contracts for the supply of back up reserves to the network with operators of power plants with a rapid start capability. Such contracts can cover months or even years but typically the time the power provider will be operating (generating power) is very short. This is illustrated in FIG. 1 which shows a typical operating profile for a storage device.
WO 2007-096656 A1 and GB1100569.1 disclose Cryogenic Power Storage Devices (CPSDs) that utilize cryogenic fluids, such as liquid nitrogen or liquid air, as storage media to store energy as thermal potential energy for providing power storage and network support services to electricity transmission and distribution networks. The Cryogenic Energy System (CES) described in WO 2007-096656 A1 is a first type of Cryogenic Power Storage Device (CPSD) and is a fully integrated storage device that requires only electricity and, optionally, heat as an input. The cryogenset described in GB1100569.1 is another type of Cryogenic Power Storage Device (CPSD) and is a simplified storage/power generation device that uses cryogenic fluid manufactured by an industrial gas liquefaction plant remote from the cryogenset, which is delivered to the cryogenset site by either pipeline or tanker. Both CPSDs described in the above mentioned patent applications benefit from receipt of low grade waste heat (hot thermal energy) from a co-located process. In addition, both devices produce low temperature cold thermal energy (cold energy) that may be of benefit to other users requiring cold e.g. air conditioning (offices); cooling (such as a data centre); freezing or refrigeration (such as a food processing factory).
However, CPSDs only operate on an intermittent basis, such as when there is high consumer demand or low generation from intermittent renewables, and typically for 250 to 1000 hours per year, and may be as low as <100 hours per year depending on the application. This poses at least three issues for the integration of the CPSD with a co-located process, which typically operates on a continuous basis. Firstly, the demand for waste heat is intermittent and so the overall utilisation of waste heat, based on the peak thermal load, will be low and therefore inefficient. Secondly, the supply of cold energy will also be intermittent and difficult to efficiently utilize in a co-located process that operates on a continuous basis. Thirdly, the rate of heat transfer from the co-located process may be different from the rate of heat transfer required for optimum efficiency of the CPSD.
These issues are also relevant to the integration of other thermal processes with one another, wherein the thermal processes to be integrated have different supply and demand criteria. The intermittency of supply and/or demand may be on an hourly, daily, weekly, monthly, seasonal or annual basis.
Thermal stores may be integrated within an energy storage device, such as a cryogenic energy storage device, in order to optimise the thermal performance. Other energy storage technologies, examples of which include but are not limited to adiabatic compressed air energy storage and Ericsson cycle technologies, may also benefit from integration with waste heat and the use of thermal storage integrated either within the storage device and/or between the storage device and a co-located process.